Trackers refer to the campaigns you create for your hashtags and keywords. They look like this:

I.e. 'smartphones' is the tracker I have created to track the keyword 'smartphones'

Whereas for posts, it is referring to the posts we have collected, within the tracker:

Here is an example of 1,012 posts collected under 'smartphones' tracker.
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The relationship between a tracker and posts is that posts are collected based on the keywords you set up for the tracker. You will have a limited amount of trackers to create within your account and a certain amount of posts you can track based on your subscription.

For example, an Individual plan only allows you to create 3 Hashtag Trackers and collect 3,000 posts per month, whereas a Team plan allows you to create 5 Hashtag Trackers and collect 10,000 posts per month.

Find out what happens when you have reached your quota limit.